The invention covers a repeater with a digital signal processing module with bandpass filtering as well as the suppression of the oscillation of the repeater as its major function.
Repeaters are frequently used to improve coverage in areas where the typical coverage of cellular networks or broadcast networks is insufficient. The general problem of repeater installation is that it requires the isolation between receiving and transmitting antennas to be higher than the gain of the on-frequency repeater to prevent it from oscillation.
In order to avoid oscillation for the repeater with the interference caused in the mobile communication or broadcast network, the repeater gain must be reduced by a gain margin with respect to the decoupling of the antennas. This safety gain margin is understood to be the difference between the gain of the repeater amplifier and the isolation of the antennas. Both measurements are determined with reference to the input and output terminals of the repeater and most frequently expressed in dB. The safety gain margin is frequently set to a 15 dB number that is determined at the time of the installation using an averaged measurement of the antenna decoupling. As this number is subject to environmental and weather conditions, the safety gain margin might need to change and follow those changes. Even with an active tracking mechanism of the antenna isolation, the operation of the repeater is limited in its enhancement and thus the repeater range and quality are the trade-off that causes the repeater installation to be less economical.
From DE 199 23 790 A1 the circuitry and the process to adaptively control the gain of an amplifier with feedback is known. Another variant of this is detailed in DE 197 52 283 A1. In the procedures described in DE 197 52 283 A1, the operation of the amplifier gain Vo is controlled in such a way that the gain margin relative to Vs, the gain of onset of oscillation, is maintained high enough to ensure a stable operation of the amplifier with feedback. The procedure does analyze the total gain which increases for a repeater operated close to the point of instability. To ensure the gain margin to be high enough for a continuous and safe operation of the amplifier, the circuitry and procedure described in DE 197 52 283 A1, the amplifier is continuously monitored and controlled.
In order to prevent the amplifier from oscillation, the difference between the regulated amplification Vo of the amplifier and the amplification for the onset of oscillation Vs shall not be reduced. In order to support same, the circuitry exhibits a memory for a pre-determined oscillation margin, which is defined by the ratio Vs/Vo.
Furthermore, the monitoring and data interpreting unit is designed in such a manner that it can determine the current safety gain margin of the amplifier from the change of the signal level at the amplifier output as a function of the change of the pre-determined amplification and compare this with the stored safety gain margin.                if the pre-determined safety gain margin is violated the pre-determined amplification Vo is lowered,        if however the current gain does not violate the safety gain margin stored and is even larger, the amplification Vo of the amplifier is raised.        
A mechanism for periodic changes of the amplifier gain can be implemented with an attenuator element for the periodic lowering of the pre-determined amplification. The pre-determined amplification of the amplifier will be lowered temporarily below the pre-determined safety gain margin during the monitoring and evaluation phase which will also reduce the probability of oscillation of the amplifier.
According to DE 197 52 283 A1, the procedure of the adaptive control of the amplification of a amplifier with feedback covers the following steps:                pre-setting the gain Vo of the amplifier;        periodical change of the preset gain Vo by a pre-determined amount;        supervising and evaluating a change of the level at the amplifier output during periodic changing of the preset gain; and        raising or lowering preset gain Vo by a second pre-determined to be operating with a gain as close as possible to the safety gain margin without violating it.        
In cellular radio networks repeater are commonly used for the extension of the coverage, e.g. in tunnels, large buildings or to supply coverage to uncovered areas, and wherever the installation of a base station is too complex.
The principle of the conventional repeater is the bi-directional amplification of radio signals in the Uplink and the Downlink direction. The radio signals remain on the same frequency as received. The Downlink signal, the signal coming from the base station of the radio network, is received with a highly directional donor antenna, amplified in the repeater, possibly filtered and re-transmitted to the mobile station via a coverage antenna. At the same time, the Uplink signal is received coming from the mobile station with the coverage antenna, is amplified in the repeater, possibly filtered and sent back to the base station via the donor antenna. The signal can be filtered either channel-selective or band-selective. Both repeater paths are usually coupled to the antenna using duplex filters. In its function to amplify, filter and re-transmit the radio signal the repeater typically introduces error (phase and amplitude errors, as well as additional noise and spurious signals), which can unfavorably affect the connection. In addition, a repeater of known design has limited dynamics: at the lower end limited by the noise of the input stages, at the higher end limited by the maximum power output power capabilities of the final power amplified stage.
In order to improve the transmission quality of the signals in repeaters, DE 196 49 853 defines a repeater for radio signals, which demodulates the received radio signals of a digital cellular radio network, transmits the data by the means of a data link (LAN, WAN) and re-modulates the data again to re-transmit the radio signal at the remote location. This repeater consists of the following functional units:                Receiver, channel filter, amplifier and demodulator for the Uplink path; Modulator and power amplifier for the Downlink path; as well as at least a data interface.        
The data interface contains substantially the following functional units: Multiplexer, Demultiplexer, digital data processing control and peripheral interface adapter.
The advantage of this type of repeater is the spatial isolation of donor and coverage antenna or the economical data line instead of a high-quality high frequency line used to connect the two antenna locations. Further advantage is that problems of the signal distortion through noise, intermodulation and amplitude or phase distortions by the digital technique can be avoided. The distance between the two partial devices of the repeater can be increased to a relatively large distance without losing signal quality of the digital transmission of the demodulated signal. Limiting factor here is only the maximally permissible signal delay.
The repeater described in DE 196 49 854 is in a similar fashion demodulating the received radio signals, processing the data and re-modulates the digital data streams for transmission. Measurements of the field strength are used as a control signal to adjust the output power of the transmission amplifier. Each repeater path covers the following functional units:                Duplex filter, preamplifier, local oscillator, mixer, channel filter, demodulator, modulator and power amplifier.        
In TDMA mobile networks (Time Division Multiple Access) the measurement of the received signal strength is carried out on a time slot basis. The demodulated digital data stream is amplified and injected into a modulator and re-transmitted with least possible errors. However, during this signal processing, no channel decoding is performed and the implementation is limited in its digital signal processing to minimize signal distortion and interference in order to substantially improve the quality of the radio network coverage. The functional units for each repeater path are a pre-amplifier, mixer, local oscillator, channel filter, demodulator, modulator and power amplifier, with the possibility of multiple paths aligned in parallel according to the number of required channels. The repeater can be remotely controlled and monitored over a radio data link established between the base station and the repeater favorably using the same signal the repeater is amplifying. This functionality is implemented by either a data modem coupled to the donor antenna or a device that is fed by the demodulated signals of the digital path inside the repeater.
From DE 196 49 855 A1 a mobile repeater is well-known. The radio signal coming from the mobile station is injected into a preamplifier after having passed through a duplex filter and mixed down into its base band or into an intermediate frequency band. The mixing frequency will be defined by a local oscillator. The base band or intermediate frequency signal is channel filtered and then demodulated providing a digital data stream. The signal is re-modulated onto a carrier frequency, raised in power by power amplifier and filtered with a duplex filter, and radiated via the coverage antenna to the mobile station. The mobile repeater further contains an intelligent control unit, which detects and analyzes signaling traffic between base stations and mobile stations, as well as the respective signal level. Thus it is possible to assign the coverage of the mobile stations to a dedicated base station of the most favorable of all possible base stations in the area and still support handover.
Beside the described channel selective repeaters, band selective repeaters are also well known. This unit filters re-transmits a whole frequency band with several channels. High selectivity values of the band filter are necessary to avoid disturbances close to the band limits. The problem of the linear repeater is now that feedback between the two antennas can lead to fatal interference or even oscillation. Therefore the antennas must be sufficiently decoupled for this type of repeater. In order to decrease the amount of feedback for the linear repeater, the two antennas have to be mounted far apart from each other which typically leads to high installation costs. In addition, the installation and maintenance costs are quite high as isolation has to be determined carefully during and periodically after the installation of the repeater with the possibility to re-adjust the repeater frequently.
Another way to implement a repeater based system for radio coverage is depicted in DE 196 48 178 A1, for which the injected radio signal is shifted to another frequency in the same radio band. In order to avoid that the terminals would not be able to successfully decode the information on the converted frequencies and to avoid the consequent erroneous reaction the modulation is changed to inverted side bands. For this the repeater contains:                two parallel input amplifiers for the input signals,        a mixer at each output of each input amplifier,        a bandpass filter at each output of each mixer,        an output amplifier at each output of each filter, the outputs of the output amplifiers being joined to generate an output signal,        at least one oscillator connected to the mixer, wherein        each mixer shifts one frequency of the input signal to another frequency within the bandpass of the system, and        the frequency position of the modulation is reflected on the frequency axis of the other frequency.        
In EP 1087559 A1, a repeater for a wireless radio network is described in more detail that uses signal processing to reduce the unwanted coupling between the output of the repeater and the input. With the means of digital signal processing, an echo signal is produced that is similar to the feedback signal between the two antennas, which is then subtracted from the signal in the main path and thus eliminating the echo signal caused by the insufficient decoupling of the antennas, so that up to a remaining error, the echo is eliminated. The digital signal processing contains in detail:                an adaptive complex filter,        a mechanism for adjustment of the filter coefficients, which exhibits a quadrature modulator for the conversion of the received signal or output signal to an equivalent baseband signal,        a FFT processor (Fast Fourier Transform), which produces an estimated signal from the equivalent base band signal,        and a DSP (digital signal processor). The DSP produces a complex impulse response from the estimated signal of the FFT processor, whereby the filter coefficients of the adaptive complex filter are adjusted in accordance with the complex impulse response. To limit the computing complexity and the convergence rate the impulse response exhibits a finite bit length/length, which corresponds to the number of filter coefficients.        
In further variation of the repeater known from EP 1 087 559 A1, a digital filter with band-pass characteristic and a mechanism for adjustment of the filter coefficients is included. The mechanism for the adjustment of the filter coefficients, which consists of a FFT processor and the DSP processor are implemented as described above. In systems incorporating BST-OFDM modulation schemes (Band Segmented Transmission Orthogonal Frequency Division Multiplexing) and/or DVB-T System (Digital Video Broadcast-Terrestrial), in which the amplitudes of the carriers of the CP signal (Continual Pilot) and/or the TMCC signal (Transmission and Multiplexing Configuration Control) is constant, the accuracy of the estimate of the transfer function increases, if a rough estimation of the transfer function is made on the basis of the CP signal and/or the TMCC signal, which is contained in all symbols of the BST-OFDM signal, and a fine estimate by means of the SP signal (Scattered pilot), which is transmitted in a certain symbol interval. The introduced delay is problematic in the repeater implementation, so that the introduced delay is significantly smaller than the repeat interval of the OFDM signals.
The previous summary of the state of the art for repeater points out, that digital signal processing is well known within repeaters. The disadvantage of such a digital repeater is in the fact that the complexity of processing and/or speed of operation are demanding requirements to ensure the impact on the signal delay to still be in the acceptable range, in particular within implementation incorporating echo compensation. Although the digital conversion promises significant improvement of the technical parameters in comparison to conventional analog conversion, the digital signal processing for bi-directional amplifiers (repeater) applications with their broad field of applications in portable radio communication and data networks as well as in the common broadcast radio technology is not yet commonly established. This is even more surprising, as both the communications technology industry and telecommunications are extremely progressive and innovative industries, where improvements and simplifications are accepted and established quickly.
The invention addresses the task to minimize the complexity and costs of a repeater with digital signal processing without trading in performance in the areas of signal filtering and echo cancellation.